SERVER-28229 makes MongoDB bind to localhost by default. However, the majority of our testing infrastructure spins up multi-node clusters on a single machine, and makes them speak to each other using the public IP address. SERVER-28229 had to add many instances of bindIp: 0.0.0.0
There may be benefits to making these nodes bind to localhost. localhost connections can incur less latency, as the kernel can bypass chunks of the networking stack. These chunks may be larger in virtualized environments like AWS.